Pervaporation method is a useful method for separating organic liquid mixtures containing volatile components by utilizing a membrane. By this method, separation of liquid mixtures of organic components which are difficult to separate by distillation because the boiling points thereof are close, because the mixture is an azeotrope, or because the organic components are denatured by heating, can be attained. This separation method has an advantage in that the energy cost is lower than separation by distillation or the like. Recently, intensive studies of this method were made especially for the separation of alcohol/water mixtures. A membrane by which water can be removed from a high concentration aqueous alcohol solution is now practically used.
In this method, an organic liquid mixture containing a volatile component is supplied to one side of a membrane and the pressure of the other side of the membrane (secondary side) is reduced with or without flowing an inert gas. By so doing, a component in the mixture having a high affinity with the polymer constituting the membrane preferentially permeates the membrane to the secondary side in gaseous form, and separation is performed. For example, in the case of aqueous ethanol solution, if the membrane has a high affinity for ethanol, ethanol preferentially permeates the membrane, and if the membrane has a high affinity for water, water preferentially permeates the membrane.
Designating the component which preferentially permeates as Component A, and designating the other component as Component B, the performance of a pervaporation method is expressed by the separation coefficient .alpha. defined as follows and by the permeation rate Q. EQU .alpha.=(PA/PB)/(FA/FB)
In this equation, PA represents the concentration by weight of Component A in the secondary side, PB represents the concentration by weight of Component B in the secondary side, FA represents the concentration by weight of Component A in the supplying side, and FB represents the concentration by weight of Component B in the supplying side. Q means the amount of the permeated component per unit area of the membrane per unit time, and the unit is usually kg/m.sup.2.hr.
In the membrane used for the pervaporation method, the thickness of the membrane largely influences the membrane performance, especially the permeation rate. That is, the thinner the membrane, the higher the permeation rate. In order to attain a membrane performance which may be practically employed, it is necessary to make the thickness of the membrane several .mu.m or less. However, it is difficult to produce such a thin membrane. Further, even if such a thin membrane can be produced, it cannot be used in practice because the mechanical strength is poor. To solve this problem, a support may be used. Thus, composite membranes comprising an active layer having the substantial separation ability by pervaporation and a support layer for mechanically reinforcing the active layer are now investigated. Such composite membranes are usually produced by first providing a porous support and then applying the active layer on the surface of the support by coating or the like.
Various membranes for pervaporation have been proposed. For example, Japanese Laid-open Patent Application (Kokai) No. 58-89911 discloses a composite membrane employing polyorganosiloxane as the active layer and polyfluorovinylidene or fluorovinylidene copolymer as the support. Japanese Laid-open patent application (Kokai) No. 60-28803 discloses a composite membrane having an active layer made of a polyethyleneimine modified by a water-soluble polymer having a dissociative acidic group, and a support made of porous polysulfone, polyfluorovinylidene, polyfluoroethylene or polyvinylchloride. Japanese Laid-open patent application (Kokai) No. 60-7801 discloses a composite membrane having an active layer made of 1-monoalkyldimethylsilylpropylene polymer and a support made of porous substituted polyacetylene, polysulfone or polypropylene. Japanese Laid-open patent application (Kokai) No. 59-55304 discloses a composite membrane having an active layer made of a reaction product of polyethyleneimine and an acid chloride, and a support made of a porous polysulfone. Japanese Laid-open patent application (Kokai) No. 59-109204 discloses a composite membrane employing polyvinyl alcohol as the active layer and porous polyacrylonitrile as the support.
The present inventors actually examined these conventional composite membranes for pervaporation method for their performance of separating a liquid mixture containing an organic solvent at a high concentration. As a result, the separation coefficients of the composite membranes other than the membrane having a porous polypropylene as the support were about 1, so that substantially no separation was observed with these membranes. It is assumed that this is because the active layer was broken due to the deformation or breaking of the support. As for the composite membrane having porous polypropylene as the support, although it excels in resistance to organic solvents, the membrane performance is very low. Thus, the conventional composite membranes for pervaporation have poor organic solvent resistance or have poor membrane performance, so that they cannot be used in practice for the separation of liquid mixtures containing organic liquid at high concentration.
As a separation membrane with reverse osmotic property used for the selective separation of liquid mixtures, asymmetric cellulose acetate membrane is industrially used. However, this asymmetric membrane has problems with hydrolysis resistance, microorganism resistance, chemical resistance, heat resistance and the like. Thus, although the membrane is practically used in some areas, it cannot be used in a wide variety of areas. New materials which eliminate the drawbacks in the cellulose acetate membrane are now actively investigated mainly in the U.S. and in Japan. Such newly proposed materials include aromatic polyamides, polyamide hydrazides (U.S. Pat. No. 3,567,632), polyamide acids (Japanese laid-open patent application (Kokai) No. 55-37282), cross-linked polyamide acids (Japanese patent publication (Kokoku) No. 56-3769), polyimidazopyrrolones, polysulfoneamides, polybenzimidazoles, polybenzimidazolones, polyaryleneoxides and the like. However, although some materials which overcome some of the drawbacks of cellulose acetate membrane have been obtained, they are inferior to the cellulose acetate membrane in the selective separation ability and permeability.
On the other hand, composite membranes having a porous membrane coated with an active layer controlling the membrane performance are developed. In the composite membranes, the best materials for the active layer and for the porous support can be independently selected, so that the freedom of membrane formation is enlarged. Further, unlike the asymmetric membranes which must be stored in wetted condition, the composite membranes can be stored in dry state. Examples of the composite membranes are described in Japanese laid-open patent application (Kokai) No. 49-133282, Japanese patent publication (Kokoku) No. 55-38164, PB Report 80-182090, Japanese patent publication (Kokoku) No. 59-27202, Japanese laid-open patent application (Kokai) No. 56-40403, U.S. Pat. Nos. 3,744,642, 3,926,798 and 4,277,344, Japanese laid open patent application (Kokai) No. 55-147106, Japanese laid-open patent application (Kokai) No. 58-24303, Japanese laid-open patent application (Kokai) No. 61-42302 and Japanese laid open patent application (Kokai) No. 55-147106.
Among these composite membranes, those having a polysulfone porous membrane and a cross-linked polyamide active layer formed on the porous membrane constitute the main stream of the composite membranes, and these are drawing attention as reverse osmosis membranes with high permeability and high selective separation ability. However, since the polysulfone support membrane employed as the porous membrane has poor heat resistance and solvent resistance, the selective separation performance is degraded if the membrane is used for the separation of an aqueous solution containing an organic solvent or for the separation of aqueous solutions at a high temperature.
There are a number of properties which a practical reverse osmosis membrane should have, such as high permeability and high selective separation ability, as well as high heat resistance and high chemical resistance. However, no membranes have been provided which simultaneously satisfy the high heat resistance and high chemical resistance. Thus, although there are membranes which can exhibit their separation ability for the separation of aqueous solutions containing organic solvents in low concentrations, if these membranes are used for the separation of an aqueous solution containing an organic solvent at a high concentration, the membrane itself or the support membrane is swelled or dissolved by the organic solvent so that the separation performance is degraded. Thus, the use of the composite membranes are limited.
On the other hand, as for the separation membrane used for the selective separation of gas mixtures, it is necessary that the membrane have a high gas permeability and high separation ability. To attain this, it is necessary that the membrane employ the composite form having an active layer formed on a porous membrane.
As such composite membranes, composite membrane having an active layer made of a silicone-based material and a porous substrate membrane made of polysulfone, polyether sulfone and cellulose acetate are known (Japanese laid-open patent application (Kokai) No. 59-12020).
However, with such a conventional composite membrane, since the resistance to organic vapor of the porous membrane is low, if the gas mixture to be separated contains vapor of a polar solvent such as toluene vapor or trichloroethylene vapor, the composite membrane is broken.